N-phosphonomethylglycine, known also by its common name glyphosate, is a highly effective commercial herbicide (available under the trade name Roundup.TM.) useful for the control of a large variety of weeds. When used in a herbicidal composition, N-phosphonomethylglycine is generally in the form of one of its various salts in solution, preferably an aqueous solution.
Many methods for producing N-phosphonomethylglycine are known including several methods for producing N-phosphonomethylglycine from aminomethylphosphonic acid (AMPA) or its salts. These methods include a process in which AMPA is added to an aqueous solution of glyoxal at 40.degree. to 45.degree. C. and heated, as disclosed in Japanese Patent Application Laid-Open No. 61992/1987; a process in which AMPA and glyoxal are reacted in the presence of sulfur dioxide, as disclosed in European Patent No. 81,459 and U.S. Pat. No. 4,369,142; a process in which AMPA and glyoxylic acid are reacted and reduced subsequently with hydrogen in the presence of a palladium catalyst, as described in European Patent No. 186,648; a process in which AMPA and chloroacetic acid are heated in the presence of an acid acceptor such as sodium hydroxide, as described in Polish Patent No. 120,759 and Spanish Patent No. 504,479; and a process in which AMPA and diethyl bromomalonate are reacted under alkaline conditions, and then hydrolyzed under acidic conditions using sulfuric acid, as described in Spanish Patent No. 545,456. These processes, however, have deficiencies, including the use of volatile gases, creation of high levels of waste by-products, insufficient product yield, relatively high materials cost, and inadequate product quality.
Other known processes for preparing N-phosphonomethylglycine from AMPA involve the hydrolysis of the intermediate material N-phosphonomethylglycinonitrile or its salts. For example, in U.S. Pat. No. 4,221,583, AMPA is reacted sequentially with formaldehyde, generally in the presence of alkali, and an alkali metal cyanide at pH 7 to 10. The resulting product, N-phosphonomethylglycinonitrile, is then converted to N-phosphonomethylglycine. The yield of isolated N-phosphonomethylglycine, based on AMPA, was only about 60% and it was necessary to use up to 2.4 times the molar equivalent of potassium cyanide and to control the pH carefully to accomplish this yield.
Similarly, in Polish Patent No. 156,933, the reaction of AMPA to prepare N-phosphonomethylglycinonitrile is sequential and multi-step using formaldehyde and an alkali metal cyanide or hydrogen cyanide. Polish Patent No. 156,933 requires even more careful control of pH by adding mineral acid in order to obtain optimum yields.
PCT/95/GB2573 is also directed to a reaction of AMPA to prepare N-phosphonomethylglycinonitrile in a sequential, multi-step process using formaldehyde and an alkali metal cyanide. PCT/95/GB2573 avoids the use of hydrogen cyanide (HCN) and also requires careful control of pH by continuously adding mineral acid to obtain optimum yields.
U.S. Pat. No. 5,453,537 further discloses a process for preparing N-phosphonomethylglycine using AMPA as a starting compound. An AMPA, in the form of its dialkali metal salt, and glycolonitrile are reacted and the resulting N-phosphonomethylglycinonitrile is hydrolyzed to produce N-phosphonomethylglycine. According to the examples, the yield of isolated N-phosphonomethylglycine was at best 78% based on AMPA. The nature of AMPA for this process requires the addition of an alkali metal hydroxide in an amount of 2 times the molar amount of AMPA for reaction to occur between glycolonitrile and AMPA. Using glycolonitrile to produce the intermediate N-phosphonomethylglycinonitrile in this process also precludes the direct cyanomethylation of AMPA by the use of more basic, more readily available, and less expensive raw materials such as formaldehyde and sodium, potassium or hydrogen cyanide.
Thus, there is a need in the art for a versatile process which directly and readily converts AMPA to N-phosphonomethylglycine and its salts in high yields from inexpensive and available raw materials.